As is known, the prior art includes disc-shaped abrasive grinding wheels having a depressed or flat center, conical, semi-flexible and exhibit an external diameter substantially comprised between 30 and 230 mm, especially used on high-speed portable electric grinding wheels, powered by electricity or compressed air (60-100 m/s peripheral velocity), also known as finishing grinding wheels, for carrying out grinding and/or cutting operations, which are essentially constituted by an abrasive mixture reinforced by armatures constituted by one or more textile meshes, on or more annular metal elements, commonly known as rings, which delimit the fastening hole of the grinding wheel to the shaft of the milling machine, and possibly by a paper label or another identifying plate adhering to one of the two faces of the grinding wheel (usually the convex face).
The abrasive mixture is generally constituted by grains of abrasive material (light green, dark green or black silicon carbide, corundum, zircon-modified corundum, semi-friable, red-brown, white, pink, ruby, ceramicated, mono-crystalline, sol-gel abrasives or sintered ceramics or others besides) having predefined particle size (normally measured in mesh) which are mixed with resins, for example phenolic, liquid and/or in powder form and possibly modified with epoxy resins, and/or others, possibly modified with organic compounds and/or vegetable or synthetic compounds, and other types of polyimide resins etc., and with additives and fillers.
The reinforcing meshes are normally glass-fiber fabrics but other types of fibers could be used, such as carbon, Kevlar or others; the textile meshes, around 1.5 m in height, are first immersed in a solution of liquid resins and solvents, squeezed between pairs of rollers and dried in appropriate ovens internally of which the resin dries without polymerizing (polymerization is then completed in the baking oven together with the baking of the grinder).
The meshes, thus-impregnated with resin and dried, are used for the blanking (or other cutting method) of the mesh discs required for reinforcing the grinding wheels.
The meshes can possibly be pre-glued to a paper sheet or a slim polymer sheet, or also to the labels.
The annular elements defining and delimiting the attaching hole of the grinding wheel are constituted by a small circular crown plate, or a plate of another shape, such as for example square or polygonal, from the internal hole of which a hollow cylindrical or non-cylindrical appendage extends; the plate adheres to one of the two faces of the grinding wheel, while the hollow appendage inserts in the hole of the grinding wheel, delimiting the internal wall.
The labels are made of paper or foil or another synthetic material and they normally have a circular crown shape (though they could have a different shape) and can occupy either the whole face of the grinding wheel or a limited area of the face to which the identifying and informative data of the grinding wheel can be attached.
Grinding wheels are produced by pressing in dies constituted by a ring in which a superiorly-open forming cavity is housed, known as a female, and by a complementary punch, known as a male.
A concave recess is formed in the central portion of the bottom of the forming cavity (the female), from which concave recess a pin rises for definition of the attachment hole of the grinding wheel; a protrusion is fashioned in the central portion of the punch (the male), which protrusion couples with the recess and internally of which a hole is afforded in which the pin is inserted during the active pressing step.
The method currently known for the production of grinding wheels consists essentially of inserting a first annular element (washer) on the pin, defining the attachment hole of the grinding wheel, the plate of the element resting on the bottom of the concave recess and the cylindrical appendage facing upwards, and of resting either a first reinforcing mesh or a suitable paper label on the bottom of the forming cavity, the reinforcing mesh being provided with glued paper having a function of supporting the mixture; further consisting in depositing a first layer of abrasive mixture, in depositing for example a second reinforcement mesh, in inserting, on the pin, an eventual second annular element (washer) defining the hole opposite the first, and in pressing, with the punch (male), at pressures in the order of 100-300 Kg/cm2 and releasing the pressed wheel from the die.
Depending on the thickness desired for the grinding wheel, successive layers of abrasive mixture alternated with supplementary reinforcement meshes can be realized on the first layer of abrasive mixture deposited.
The first mesh placed, i.e. the lowest mesh or the “backbone”, the mesh which in the case of a depressed-center wheel, is located on the outside of the grinding wheel, can be placed directly on the bottom of the forming cavity of the matrix coupled to a sheet of paper material, or by interposing an annular paper label or a like element between the bottom of the forming cavity and the first mesh.
The label or the bottom of the forming cavity performs the function of containment and support of the abrasive mixture deposited internally of the forming cavity, and in order to perform this function, must exhibit an adequate rigidity.
An example of known abrasive wheel is shown in the prior document JP S50 23178, in which the lowest mesh, which is the first reinforcement mesh, has links equal in size to the links of the other reinforcement meshes (i.e. the second reinforcement mesh), therefore the amount of abrasive mixture that encompasses the rear of the first reinforcement mesh and which encompasses the second reinforcement mesh, adjacent to the first, is very limited and not very resistant.
The pressed grinding wheel once removed from the die is subsequently subjected to heating at a temperature slowly rising from 80° C. to 125° C.; in these conditions the resins of the abrasive mixture and the resins impregnating the reinforcement mesh or meshes become fluid, “merging” together and “interpenetrating”; in this way the mixture adheres to the mesh or meshes and together with them creates a single block.
A subsequent re-heating of up to 180-190° C. (but even to lower temperatures) determines the process of irreversible polymerization of the resin.
These known methods for the production of grinding wheels and the grinding wheels obtainable with the method are not free of drawbacks.
A first drawback consists in the fact that, in a case of production of depressed-center grinding wheels, during the depositing of the abrasive mixture internally of the cavity, at least the central portion of the first reinforcing mesh or the labels rested on the bottom of the cavity, which portion is at the concave recess of the cavity itself, yields and flexes below the weight of the abrasive mixture deposited, which is a cause of undesired variations of density and thickness of the central portion of the grinding wheel with respect to the peripheral portion thereof.
A further drawback of the known methods consists in the fact that the adhesion of the first reinforcing mesh, the one deposited on the bottom of the forming cavity, to the abrasive mixture, is limited and incomplete; this is due both to the fact that the abrasive mixture adheres to a single face (the upper face) of the first reinforcing mesh or, at most, penetrates into the mesh openings and glues the flanks of the mesh wired, leaving the face resting on the bottom of the cavity and/or on the label uncovered, and also is due to the very limited adhesive properties of phenolic or phenol-epoxy resins normally used.
This drawback is most felt in a case where the first reinforcing mesh is a mesh glued to a slim paper sheet; in this case, the sheet obstructs the spreading of the abrasive mixture to below the links of the mesh, preventing the mesh from sinking into the grinding wheel.
Known grinding wheels exhibit a further drawback consisting in the fact that, particularly when used for grinding operations, the upper edge thereof (the convex side or in any case the back side) is subject to irregular and excessive wear due to the over-stress it is subjected to (vibration, shock, non-planar contact). This drawback is even more clearly noted in grinding wheels of limited thickness, of the order of 3-4 mm (cutting and grinding wheels), the flexibility of which, in fact, exacerbates the over-stress when used in grinding.
It has been found that by using layers of abrasive mixtures of different particle sizes, it is possible to improve resistance to over-stress of the edge; this is done by using, for the first layer of abrasive mixture deposited in the die (the backbone layer, convex or planar, depending on the grinding wheel), mixtures with fine abrasives (36-46-60 mesh) and high-resistance resins.
It has also been also found that it is sufficient to sink and immerse the first mesh, the mesh resting on the bottom of the forming cavity of the die, internally of the first layer of deposited abrasive mixture, even if only for a thickness of less than 1 mm, so as to obtain a significant improvement in the resistance to stress of the upper edge of the grinding wheel.
This operation, however, has been found to be particularly delicate.
A method aimed at achieving this result could be to deposit, on the bottom of the cavity of the forming die, a thin layer of abrasive mixture on which to rest the first mesh.
However, given the limited thickness involved, this method is practically difficult to implement and difficult to control, particularly in the case where abrasive mixtures have to be used which contain coarse abrasives, and does not guarantee the repeatability and constancy of results, and requires an expenditure of time and resources that bear heavily on production costs. In addition, there is always the problem of supporting the material at the position of the concave cavity.
To obviate the above drawbacks the present applicant proposed a solution described in Italian patent no. IT 1 334 480.
In particular, to obviate the problem of incorporating the first reinforcing mesh, namely the “backbone” reinforcing mesh, in a layer of abrasive material, even of only a limited thickness, a production method of the grinding wheel was developed that that included following steps:
inserting a support element on a pin for forming an attachment hole of a grinding wheel emerging from the forming cavity of a die matrix, wherein the support element comprises a plate in which a through-hole is defined for insertion on the core and is provided with a first face intended to at least partially rest on the bottom of the cavity, and an opposite second face where projections rising from the second face are defined, which have a predefined height;
laying, internally of the cavity, at least a first reinforcing mesh so that so that it rests on the rising projections of the plate and remains separated from the bottom of the forming cavity at least at a height equal to the height of the projections;
depositing a predefinable quantity of abrasive mixture which incorporates the first reinforcing mesh suspended from the bottom of the forming cavity, and
pressing the support element, the first reinforcing mesh and the layer of mixture so as to obtain the grinding wheel.
However, though widely used and with excellent results, even this solution is not without drawbacks, the first of which is certainly the cost of the preparation of the suitably-shaped support elements provided with the projections.
A further drawback encountered in this solution is the fact that the reinforcing mesh is supported at discrete points thereof in the central area; therefore the periphery of the mesh, under the weight of the abrasive mixture before it penetrates the spaces between the meshes of the first reinforcing mesh and goes to rest on the bottom, defining a support layer for the mesh, can flex and thus lose the substantial planarity thereof.
In order to limit this drawback, the bottom of the forming cavity of the die is machined to define respective projections on which the reinforcing mesh rests; this not only increases the cost of the die, but also defines cavities in the finished grinding wheel.
An aim of the present invention is to obviate the above-mentioned drawbacks in the prior art, with a solution that is simple, rational and relatively inexpensive.
These aims are attained by the characteristics of the invention reported in the independent claims. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.